Hello Stefan,
Thanks for using our plugins and for reaching out!
We are unfortunately unable to provide support through our Pre-Sales forum channel, but you can head over to The Events Calendar WordPress.org support forums for assistance and answers.
If you have a PRO license, you can log into your account and get assistance through the PRO forums. If you need to reset your password, you can do so here. This will send an email to the account used when you initially registered for the support forums.
To get you started on this, with CSV import at the moment the only way to compare 2 events is through the event title and event date. If these are the same for 2 events then the events are considered the same. On our long-term roadmap we have some plans to further enhance this functionality.
What you can do meanwhile is use a slightly different title, for example insert an extra space somewhere.
Thanks for understanding and good luck resolving your issue. Have a good day!
Cheers,
Andras